The present invention relates to a method and technique which imposes an off-set and thus stresses an optical fiber for performing an accelerated proof test of the optical fiber.
In the prior art, methods are known for testing optical fiber, such as three and four point bending, tensile loading, and two point bending. Such methods suffer from various problems; for example, in three point bending, it is possible for a flaw to be located in an area having a small bending moment, while an area having the largest bending moment may not contain a flaw. The results of such a test may therefore indicate a reliable fiber when, in fact, the fiber may be unreliable. Tensile loading may be undesirable in that one end of the fiber must be affixed to a surface, and weights are attached to the other end of the fiber. Two point bending may also be disadvantageous, since this method focuses on a small area of a fiber, and then only one side of the fiber is in tension.
Thus, a need exists for alternative methods for testing longer portions of optical fibers, which also do not suffer the problems of such testing methods in the prior art.
Optical fiber interconnects have been considered in the prior art which have been clamped at the ends and then subjected to not-very-small lateral ends off-set. The term "not-very-small" is defined herein to mean that the off-set is sufficiently small to apply elementary or linear beam theory to predict the stresses and strains for the given off-set, but also that the off-set is large enough so that the reactive tension is appreciable. Such reactive tension occurs because of an inability of the interconnect ends to move closer during its bending.
If the reactive tension is large, and so is the tensile stress, one has to consider the non-linear stress-strain relationship of the silica material in order to make a sufficiently accurate prediction of the induced stress. It has been found in the prior art that optical glass fibers subjected to tension exhibit a non-linear strain relationship. This can be described by the following expression: ##EQU1## in which .sigma. is the applied stress; .epsilon. is the corresponding strain; E.sub.0 is Young's modulus at low strain values of the optical fiber material (say, silica-based materials); and .alpha. is a parameter specifying the degree of non-linearity. Typically, E.sub.0 =10.5.times.10.sup.6 p.s.i. and .alpha.=6.
Accordingly, for a not-very-small lateral ends off-set of an optical fiber interconnect, the effect of the non-linear stress-strain relationship on the induced tensile strain/stress may be significant, and so such non-linear effects in the optical fiber interconnect should be accounted for in stress/strain measurements and designs.